An informal reply to my English-speaking friend Adam.

Hi!, Adam. It looks odd with that exclamation point. Use a comma instead.

Congratulations for on winning a mountain bike on that Internet online auction. I would love going out for a ride together this weekend. It's not wrong to use "Internet", but "online" sounds more natural.

I would prefer to go out on Saturday, because I have to meet with my grandparents on Sunday.

Yes, you're right when you say that it's much more fun to ride off-road; but where can we go so should we go? And how far?

Of course that since we're going to spend all day riding out into the countryside and I really think we should take food with us. I could tell to ask my mother wheather if she could prepare something tasty for us. You did spell "whether" wrong here, but it's better to use "if". Or, you could use "whether or not". 

Please reply to me soon!

All the best, Make sure to remember your double consonants. It's "all", not "al". :)

Álvaro.

I hope you guys have fun!

   [Dear] Adam, 

   Congratulations on winning the mountain bike. I would like to go for a ride with you this Saturday; I will be with my grandparents on Sunday.

   Yes, I agree that it's much more fun to ride off-road, but where and how far should we go? Since we're going to ride all day in the countryside, I think that we should bring food. I could ask my mother to prepare something for us to eat. Please reply soon. 

All the best,

Álvaro.

An informal reply to my English-speaking friend Adam.

Hi! Adam.

Congratulations for winning a mountain bike on that Internet auction. I would love going out for a ride together this weekend.

I prefer to go out on Saturday, because I have to meet with my grandparents on Sunday.

Yes, you're right when you say that it's much more fun to ride off-road; but where can we go so? And how far?

Of course that if we're going to spend all day riding out into the countryside, then I really think we should take food with us. I could tell to my mother so then she could prepare something tasty for us.

Please reply me soon!

All the best,

Álvaro.